The present invention relates, in general, to integrated circuits and, more particularly, to an apparatus, method and system for providing common-mode stabilization, such as through feedback of a common-mode voltage, in integrated circuits having differential operation.
Differential operation of integrated circuits (xe2x80x9cICsxe2x80x9d) such as amplifiers, operational amplifiers (op amps), and filters, is increasingly common. Such ICs having differential operation, referred to herein as differential circuits, operate utilizing two different input signals, a first or positive input signal, and a second or negative input signal (which is an inverse or 180 degree phase shifted version of the first input signal), and provide two output signals, a first (or positive) output signal, and a second (inverse or negative) output signal. Such differential circuits generally provide greater noise immunity compared to non-differential (or single-ended) circuits. For example, common-mode disturbances are rejected, as common-mode disturbances are generally equally coupled to the corresponding two outputs, and in a differential operation in which one output is subtracted from the other output, such common-mode disturbances are cancelled. Such differential operation, as a consequence, tends to reduce noise contributions both from substrate noise in the switching of ICs and from power supplies.
Differential circuits have additional advantages, such as superior linearity compared to non-differential circuits, due to suppression of even-order distortion products. Differential circuits are also comparatively faster, avoiding xe2x80x9cmirrorxe2x80x9d poles in the signal path, and generally also have comparatively greater voltage swings.
In differential circuits, however, the positive and negative signals fluctuate or swing about a voltage midpoint, which must be maintained within a particular range for proper circuit operation. This voltage midpoint is typically defined through the use of additional circuitry, by detecting a common-mode voltage level (VCM) of the two (positive (V+) and negative (Vxe2x88x92)) output signals, defined as one-half of their sum, i.e., VCM=(V++Vxe2x88x92)/2. This common-mode voltage is detected, compared to a desired or reference value of the common-mode voltage level, with negative feedback utilized to correct any error in the detected common-mode voltage (i.e., to minimize any difference (error) between the reference voltage level and the detected common-mode voltage level.
Prior art methods generally utilize a comparatively high gain amplifier in the feedback circuit or path, to minimize the common-mode voltage error. Use of such high gain amplifiers, however, may result in stability issues due to a comparatively large number of poles. In addition, such amplifiers must operate at least as fast as the signal variation, further complicating the circuit design, requiring a high bandwidth, and generally consuming significant power.
As a consequence, a need remains for an apparatus, method and system which provide accurate common-mode voltage feedback without requiring high gain amplification in the feedback path. Such an apparatus, method and system should also be particularly suited for high bandwidth applications, and should provide comparatively low power dissipation.
A system, method and apparatus are disclosed for common-mode voltage feedback. The preferred system of the present invention includes a plurality of differential circuits, a corresponding plurality of common-mode voltage detectors, a corresponding plurality of buffer circuits, and one common-mode control circuit. Each differential circuit is operative to produce a first differential output voltage and a second differential output voltage. Each corresponding common-mode voltage detector is operative to provide a common-mode voltage from the first differential output voltage and the second differential output voltage. The common-mode control circuit provides a control voltage signal from the common-mode voltage and from a reference voltage. Each buffer circuit is operative to adjust the corresponding common-mode voltage using the control voltage signal to provide a common-mode feedback voltage signal to the corresponding differential circuit.
In the preferred embodiment, each buffer circuit is implemented as a source-follower circuit, and preferably as multiple stages of source-follower circuits. These buffer circuits replace the high gain amplifiers typically employed in the prior art.
One common-mode control circuit is utilized for the entire system. The common-mode control circuit provides the control voltage signal to each of the buffer circuits, to control the common-mode feedback voltage provided to each corresponding differential circuit.
This use of the buffer circuits with a common-mode control circuit of the present invention, in lieu of high gain amplifiers, provides for a significantly improved frequency response, and does so without the corresponding complexity of high gain amplifiers. Second, this use of the buffer circuits with a common-mode control circuit of the present invention provides for important power savings, significantly reducing power dissipation of the differential circuit ICs. In addition, the buffer and control circuits of the present invention may be implemented in a wide variety of designs and implementations, with reduced complexity, reduced chip area requirements, and corresponding fabrication efficiencies.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.